varanid lizards (genus Varanus), which include the Komodo dragon (Varanus komodoensis), are among the most adaptive reptile species on earth. Scientific explanation of the diverse varanid behavior (44 species spread across Australia, Asia, and Africa) relies on an understanding of their cardiac physiology (5). As a subject of comparative physiology, the varanid lizard is unique. Within physiology circles, the varanid lizard is noted for a remarkable aerobic scope by reptilian standards (7). Support of high metabolic rates necessitates a physiologic infrastructure adapted for cardiac performance as well as oxygen transfer (2). Cardiac performance in reptilian species is particularly interesting in that both systemic and pulmonary perfusion are served by a common ventricle (1). Ventricular morphology of varanid lizards, by contrast, features a muscular ridge enabling both high-pressure systemic, and low-pressure pulmonary, perfusion. These atypical anatomic/physiologic features largely explain this lizard's gross cardiovascular adaptations that permit a robust metabolic capacity needed to support its unique behavior. However, the underlying cellular mechanisms of interest in varanid species are not fully understood.
Varanid lizards are collectively termed “monitor lizards” for their ability to monitor the surroundings while supported by their back legs. Vertical monitoring by an otherwise horizontal creature highlights the need for rapid accommodations in cardiac function. Similarly, the predatorial nature of many varanid species serves as a functional example in which acute alterations in cardiac performance underpin the broad scope of their aerobic capacity. The varanid species are also thought to possess efficient thermoregulatory capacities (for ectotherms) due largely to subtle alterations in heart rate. For instance, heart rate hysteresis enables rapid heat acquisition during sun bathing, while reverse hysteresis preserves core temperatures in the shade (6). Indeed, cardiac adaptability for thermoregulation is essential to varanid species that exist in a variety of potentially harsh habitats including old world tropics, subtropical, and arid regions (5).
Not surprisingly, current efforts to understand varanid cardiac physiology have transitioned from systemic and organ-based characterizations to an understanding of cellular mechanisms of cardiac regulation. Recent findings suggest that the high heart rates and blood pressures observed in varanid lizards may be due to well-adapted excitation-contraction (E-C) coupling. By reptilian standards, this somewhat mammalian-like cardiac cycle may rely on calcium derived from the sarcoplasmic reticulum (4). In Galli, et al. (3) findings from electrophysiological experiments performed on isolated varanid cardiomyocytes confirm a regulatory role of the sarcoplasmic reticulum in E-C coupling. A novel finding of this investigation demonstrates that, while cardiomyocytes are structurally similar to those of reptiles, the cardiac cycle is predominantly controlled by extracellular calcium. A major source for this calcium appears to be the Na+/Ca2+ exchanger (3).
In total, these findings from Galli et al. (3) shed new light on the interesting topic of varanid cardiac physiology. The relative contributions of the sarcoplasmic reticulum and extracellular calcium to E-C coupling in the varanid lizard are important for understanding its relatively mammalian-like cardiac performance and high aerobic capacity. At the same time, these findings also support the well-conserved reptilian facets of varanid cardiac physiology, necessary for species survival during seasonal climate alterations and periodic food shortages.
- Copyright © 2009 American Physiological Society